This invention relates to MEMS circuits and, more specifically, to a method and circuit for testing the characteristics of the individual capacitances of a MEMS sensor that has been sealed or packaged that may be integrated with another circuit or packaged with another circuit.
One prior art model of a MEMS sensor integrated with a circuit is shown in FIG. 1. The MEMS sensor 100 is represented by a first capacitor 110 and a second capacitor 120 sharing a common node 130a that represents a moving element. When an excitation voltage is applied to the plates 110a and 120a of a MEMS sensor and fluctuations occur on the moving element 130a due to an input stimulus like acceleration or pressure, the moving element changes its position according to the input stimulus. When this occurs, capacitances between 110 and 120 change. The output 130 of the moving element of the MEMS sensor 100 is fed into a first amplifier input 141 of a capacitor-voltage (C-V) converter 143. The other input to the amplifier is connected to a reference voltage 142. During a reset, the reference voltage is also applied to the two plates 181 and 182 of the MEMS sensor 100. The reference voltage 142 can be hardwired to the sensor and amplifier directly through the pins of the package or can be controlled by an on-chip control 150, such as an ASIC or other control logic. In either case, after the packaged MEMS sensor and IC have been reset by switch 192, the voltage applied to the plates of the sensor is excited by changing the voltage directly applied to the pins of the package or by programming the control logic to switch between various voltages supplied to the package. The excitation voltage applied to plates 181 and 182 start at the voltage reference after reset and then are excited to an excitation voltage that is equal in magnitude and opposite in polarity. For instance, the voltage applied to the first plate 181 would step from the voltage reference to an excitation voltage (Vexcite) at the same time that the voltage applied to the second plate 182 steps from the reference voltage Vref to a negative excitation voltage (xe2x88x92Vexcite). Each step function would then alternate to its original reference voltage state and back again so that any fluctuations on the moving element 130a would cause corresponding fluctuations on the capacitors 110 and 120.
The amplifier 140 produces a C-V output voltage 155 reflecting the difference between the first and second capacitances 110 and 120 experienced by fluctuations in the moving member caused by the input stimulus. The C-V output voltage 155 is typically modified by a feedback capacitance Cref represented by feedback path 145 (and reset by switch 192) such as to produce an output voltage Vout=xe2x88x92[(C1xe2x88x92C2)/Cref]*(Vexcitexe2x88x92Vref). The C-V output voltage 155 is then signal conditioned as needed by other integrated circuitry 160, such as filters, gain and offset trim and the like. The final output voltage 170 of the integrated device represents the physical activity of the MEMS sensor and is used in various applications such as accelerometers, pressure sensors, gyroscopes.
To reduce failure rates, the MEMS sensors are tested before being packaged. However, before and during packaging with the circuit, additional processing problems cause some MEMS sensors to malfunction or become damaged. Some problems may occur due to moisture ingress into the capacitor, for example. Common MEMS problems involve stiction where the moving element 130a or proof mass comes into contact with the fixed plates 181, 182. Additionally, breakages or holes may occur in the moving element. Because the MEMS sensor""s moving element is extremely delicate, often resulting in capacitance changes in the few femto-farad range, direct connections to sensor elements such as the moving element are problematic as any probing would make the measurements inaccurate. Additionally, once the circuit and sensor are packaged, the only measurement available always reflects the difference between both sensor capacitances and does not assist in identifying problems with the individual capacitances.
Accordingly, what is needed is a packaged sensor device that allows for more accurate testing of the MEMS sensor after it has been packaged with an IC.